EVALUATION OF METAL OXIDE NAOSTRUCTURES FOR APPLICATION IN PHOTOCATALYSIS USING FIRST PRINCIPLES MODELING.
Dr.chem. Jurijs Žukovskis (ISSP UL, Laboratory of Computer Modeling of Electronic Structure of Solids)
In the framework of the international ERA.Net RUS Plus project, the zone structures of metal oxides’ (SrTiO3,TiO2 un ZnO) nanotubes and nanowires (in the band gap range) and their relation to optical excitation spectra in the visible light range (1.5-2.8 eV). This is important for the dissociation of water molecules in an electrochemical system that includes nanoelectrodes and electrolyte solution. The width of the metal oxide band gap corresponds to the ultraviolet light range, its role as the photodissociation of H2O molecules in visible light is not possible if the electrode materials are characterized on a volume scale. In addition, nanoelectrodes in their pure form do not provide for their application in photocatalysis. Only by modifying metal oxide nanostructures with certain point defects, incl. impurities, we can narrow the width of the band gaps to the visible light range.
In addition, corresponding band gap edges for selected nanostructures must match with the so-called reduction (H+/H2) and oxidation (O2/H2O) potentials (4.44 eV and 5.67 eV, respectively), which should be located in the respective band gap Δεgap range. For example, if we evaluate TiO2 anatase-type (101) and (001) oriented nanotubes for photocatalytic applications, it is their doping with point CO, FeTi, NO un SO impurities or NO+SO dopant dimers that leads to Δεgap reduction from 3.2 eV in titanium dioxide volume (with ~1% efficiency solar spectrum energy conversion). In contrast, in NO and SO-doped TiO2 nanotubes, due to defect levels in the band gap, their width reduces up to 2.4 - 2.5 eV with ~15% solar spectrum energy conversion. The difference between the edges of the band gaps (CB bottom of the control area and the valence area of the VB peak, respectively) for higher occupied and lower unoccupied levels in doped nanotubes induced within the same interval according to the correct distribution of these levels depending on reduction and oxidation potentials, could be described by the following inequality: εVB < εHOIL < εO2/H2O < εH+/H2 < εLUIL < εCB.
We analyze the applicability of large-scale calculations of electron structures from first-principles doped SrTiO3, TiO2 and ZnO nanotubes and nanowires for the application of these materials in photocatalysis reactions. The relevant models and calculation methods are also analyzed.